Polydimethylsiloxane (PDMS) based microdevices have bought revolutionary change in the chemical analysis. Suitable mechanical properties of PDMS make it amenable for soft lithography and certain optical properties like transparency in Near InfraRed (NIR) make it a yet unexplored option for use as a window material in Fourier Transform InfraRed (FTIR) Spectrometers. In our work, we investigate the utility of PDMS as a transmittance cell kit in FTIR Spectrometer replacing the CaF2 windows that are currently in use.
In this work, the spectral characteristics of PDMS in NIR and MIR have been thoroughly investigated. A set of design parameters were extracted through experiments and a preliminary design for a PDMS based cell kit has been suggested. PDMS Cell kit was fabricated using a SU-8 coated silicon wafer mold using soft lithography. Experiments by varying the base to curing agent ratio of PDMS pre polymer as well as selected heat treatment protocols were conducted, to improve the transmittance of PDMS in the MIR region. Due to the variation in base transmittances of PDMS across different samples and also due to inherent variations introduced by the FTIR spectrometer, there has not been an appreciable change in the optical characteristics of PDMS.
Further, to prove the utility of PDMS as a window material in the NIR region, Samples of Extra Virgin Olive Oil (EVOO) were analyzed with the PDMS cell kit and the results were compared with the spectral analysis on same samples with a CaF2 kit. Even though PDMS does introduce its own artifacts, if a critical sample volume is used for the analysis, the performance of the PDMS kit is comparable to that of CaF2 cell kit. This volume which is dependent on the ratio of the thickness of a particular sample to that of the PDMS slabs, and the Critical Volume Ratio (CVR) of EVOO have been identified through this work. Once the CVR for a particular test sample is known, a PDMS based cell kit can effectively replace a CaF2 kit and hence will prove as a less expensive option. This opens up multiple opportunities for enabling low cost and rapid liquid sample analyses using PDMS based microfluidic devices directly as window material in an FTIR spectrometer.